The present invention relates to polymer extrusion and injection molded products and processes. The invention has particular application to a polymer extrusion or injected molded polymer that includes an addition of wood flour as an ingredient to the composite material.
As a result of the increasing cost of wood and the attendant cost of wood frames, there is a demand for alternatives to solid wood construction products. Accordingly, the use of plastic extrusion profiles as substitutes for wood products in the area of picture frames and door, wall and window frames and moldings has increased in recent years. Injection molded plastic has also been utilized in the furniture industry for forming plastic chairs, lounges, tables and the like.
Polymer extrusions are well-known for use in forming components for window and door frames. Plastic extrusion profiles are commonly formed with polymer materials such as polyvinyl chloride and chlorinated polyvinyl chloride (PVC and CPVC), polystyrene, acrylonitrile-butadiene-styrene (ABS) or similar materials. Polymer materials are conventionally used in combination with a variety of fillers, lubricants and pigments. Additionally, for foamed polymers, a blowing agent is also added to the composite. An example of a foamed extrusion is found in commonly assigned U.S. Pat. No. 5,508,103, entitled "Extrusion Product with Decorative Enhancement And Process Of Making The Same."
As a result of the increasing cost of polymer materials, there exists a need for less expensive filler materials, which can be used with polymers such as PVC, but do not adversely affect the strength or wear resistance of the resulting profile. One such filler material that has been given increasing consideration is wood flour or wood fiber. Polyvinyl chloride thermoplastic polymer materials have been combined with wood and wood fiber to make extruded or injection molded materials.
Various patents and publications have proposed extrusion and injection molded materials that combine polymers and wood fibers. One example of such a patent is U.S. Pat. No. 5,406,768, assigned to Andersen Windows. The '768 patent describes a structural component made of polymer and wood fiber in which the polymer is rigid PVC. While rigid PVC has many uses, the present invention is directed primarily to the use of a composite with foam PVC. The extrusion of foamed PVC, includes a blowing or foaming agent. Addition of foaming agents can also affect the ability to add wood flour fillers. However, the use of foamed PVC has advantages over rigid PVC in that the less-dense foamed material allows substantial cost savings in the amount of PVC used in the composite. U.S. Pat. Nos. 5,082,605 and 5,088,910 also describe composite materials formed from polymers and wood or cellulose fibers.
One difficulty presented by additions of wood flour is the absorption of moisture by the wood flour. The loss of moisture within the extrudate can slow the extrusion rate as a result of an increase in viscosity. Another difficulty involves foaming or expansion of the extrudate with the wood flour. The wood flour particles do not expand or foam and, thus, the foaming must be done around the wood flour particles. Also, unlike polymers such as PVC, the addition of heat to wood flour does not improve flowability of the extrudate.
The present invention overcomes these difficulties by a two step process in which the wood flour particles are encapsulated by the resin and pelletized. The pelletized pieces are then mixed with additional resin and blowing agents. It is believed that the encapsulation process prevents moisture absorption and permits foaming of the extrudate. Additionally, the encapsulation provides a smoother skin in the final product.